qqnorm and qqline in ggplot2

Question

Say have a linear model LM that I want a qq plot of the residuals. Normally I would use the R base graphics:

qqnorm(residuals(LM), ylab=\"Residuals\")
qqline         


        

Answer 1

The following code will give you the plot you want. The ggplot package doesn't seem to contain code for calculating the parameters of the qqline, so I don't know if it's possible to achieve such a plot in a (comprehensible) one-liner.

qqplot.data <- function (vec) # argument: vector of numbers
{
  # following four lines from base R's qqline()
  y <- quantile(vec[!is.na(vec)], c(0.25, 0.75))
  x <- qnorm(c(0.25, 0.75))
  slope <- diff(y)/diff(x)
  int <- y[1L] - slope * x[1L]

  d <- data.frame(resids = vec)

  ggplot(d, aes(sample = resids)) + stat_qq() + geom_abline(slope = slope, intercept = int)

}